244 



EXPERIMENTS ON MARIOTTE'S LAW. 



closed end, and mark these distances by a small strip of paper 

 pasted upon the tube, as shown in fig. 167. Fill the tube with 

 mercury up to the mark near the open end, thus leaving in it a 

 column of air 10 long, under the pressure of the atmosphere. 

 Close the end with the finger, invert the tube, and immerse the 

 lower end in a vessel containing mercury, precisely as in the 

 barometer experiment. Remove the finger and incline the tube 

 until the top of the mercury stands at the upper mark. The 

 volume of the air in the tube is now doubled, for it is 20 cm long, 

 and the perpendicular height of the mercury in the tube will 

 now be represented by the length a 6, which will be found to be 

 half the height of the barometer at the time, that is, the mercury 

 stands twice as high in a tube having no air at the top, as in the 

 tube having air above the mercury; this air therefore evidently 

 exerts a pressure which is half of that of the atmosphere, that 

 is, the pressure is diminished to one half, when the volume has 

 become twice as great as it was originally. 



Draw out one end of a dry glass tube, 6'0 cm or 70 cm long, and 2 mm 

 wide, into a fine aperture. Immerse the other end in mercury and, 



taking care that no moisture enters the tube, suck up 

 into it a thread of mercury equal in length to about half 

 the average height of the barometer ; if the latter is 

 74 cm , make the length of the thread of mercury 37 cm . 

 Move the thread, by cautiously tapping or inclining 

 the tube, until its end is about 10 cm distant from the 

 fine aperture, as in fig. 168 A, and close the end 

 with the blowpipe, taking great care to direct the 

 flame well across the end to be closed, and not upon the 

 tube itself, so as not to heat the air in it, which would in 

 that case expand and partly escape from the tube before 

 it is closed. To maintain the thread in the required J 

 position, place the tube upon a table, let the fine aper- | 

 ture project about 10 cm from the edge, take the lamp ] 

 in your left hand, the blow-pipe in the ri^Iic, and closr 

 the end without moving the tube. 



When cool hold the tube in a vertical position, the open end 

 being first at the top, fig. 168 J5. The enclosed air will now 

 be compressed to two-thirds of its previous volume, for in this 

 position it lias to bear, in addition to the pressure of the 



FIG. 168 

 (real sice). 



